Pressure sensitive adhesives (PSA) have been used in a variety of applications, as they provide many desirable characteristics such as removability and ease of application. For a more permanent and highly smooth bond, some conventional PSAs may not necessarily have sufficient strength to hold and maintain its adherence on certain substrates. Furthermore, a conventional PSA when applied to certain materials, may not be able to withstand exposure to elevated temperatures or high humidity. For example, application of a PSA on acrylic sheets and polycarbonate sheets that are known to be “out-gassing materials” and difficult to bond, can result in bubbling and delamination.
Curable adhesives (e.g. heat or light cured) have been used in applications where substrates require substantial permanency and high strength adherence. Conventional curable adhesives, however, are typically not provided as a PSA nor in the form that is easy to apply, such as a tape. For optical product applications (e.g. glazings), curable adhesives have been desirable, as they can provide optically clear, strongly adhered laminates (e.g. layered substrates).
To achieve both strength and ease of application, hybrid compositions have been developed that can be used in optical applications. For example, a light curable, polyester based adhesive has been used for plastic glazing applications. In digital video disc (DVD or optical discs) bonding and CRT applications, a liquid adhesive formulation has been used. For bead bonding in making retroreflective articles, a curable polymeric network has been suggested.
Strength and application, however, are not the only criteria that many optical substrates/laminates require. Certain optical products are exposed to harsh environmental conditions, such as heat, UV (solar) light, water, etc. For example, vehicle windshields generally exist in outdoor conditions that submit them to all types of weather. These windshields typically include substrates such as acrylic or polycarbonate, adhered to a solar or infra-red (IR) reflecting film made from a multi-layer optical film (MLOF) (3M Co; St. Paul, Minn.). The materials may become optically obstructed if the adhesion between the layers is damaged or compromised.
What is desired is an adhesive composition that can be used in applications where optical clarity is needed, as well as ease of application for efficient manufacturing. An adhesive composition whose integrity is maintained even when exposed to extreme temperature and moisture conditions is also desired.